i'm just gonna plop down an enormous list:
Death, Carcass, Soilwork, At The Gates, Defleshed, Corporation 187, The Haunted, Shadows Fall, Opeth, Moonspell, Dimmu Borgir, Sentenced (early albums), Darkane, God Forbid, Carnal Forge, Gardenian, Emperor, Vintersorg
those are all "extreme," as the term is loosely defined.... black/death/thrash/whatever
Nevermore fucking kicks ass too, but they're more of a trad/power sounding band.. and Iced Earth if you can tolerate the predictability
just d/l a bunch of shit from that list until you find something you like..